1. Field of the Invention
The present invention is directed to providing cured cyanate ester composites with a conductive metal surface, and more particularly, to applying a film of cyanate ester resin to the surface of a cyanate ester composite to achieve better adhesion between the composite and a subsequently-plated metal.
2. Description of Related Art
Metals are commonly employed in manufacturing because they offer high degrees of ductility and strength as well as high conductivity. However, metals are generally heavier than other common materials like plastics, such that the positive characteristics attributable to metals often come at the cost of increased product weight. Increased product weight is particularly a concern in industries manufacturing vehicles of transport such as automobiles, aircraft, and spacecraft, as well as payloads of such vehicles, given that increased weight adversely affects fuel economy. Accordingly, these industries have increasingly incorporated non-metallic, lighter-weight materials such as plastics into automobiles and aircraft in an effort to economize fuel.
However, plastics are not universally suitable as substitutes for metals. For example, while plastics offer high degrees of ductility and strength, plastics are relatively nonconductive materials. Thus, plastics cannot supplant metals used as electrical, thermal, or microwave conductors.
It would therefore be desirable to plate a metal coating onto plastic, thereby simultaneously realizing the benefits of both metals and plastics. More specifically, metal plating on plastic materials allows the use of these lighter-weight plastic materials for the bulk of components and minimizes the amount of metal required to achieve a highly conductive surface. Of particular interest is metal plating on cyanate ester polymer composites of cyanate ester resin and graphite, since such composites have certain unique advantages that make them very useful for specific applications, such as applications in communications spacecraft. Specifically, cyanate ester polymer composites can be formulated in ways that make them very resistant to even minute dimensional changes that would otherwise occur as a result of temperature changes or the absorption and desorption of moisture in the presence of air. However, plating metal onto plastic materials, such as cyanate ester polymer composites, has been challenging because these materials do not inherently adhere to one another.
Two methods have been developed by which a cyanate ester composite surface may be treated in preparation for the subsequent plating of metal, both of which are the subjects of previously-filed applications assigned to the present assignee. An application entitled "Preparation of Cured Cyanate Ester Resins and Composites for Metal Plating" (Ser. No. 08/339,390), filed on Nov. 14, 1994 recites treating the surface of cyanate esters polymers and composites with a preheated solution of an alkali metal salt of an alkoxide to achieve greater adhesion between the surface and subsequently-plated metals. A second application filed on even date therewith and entitled "Preparation of Cyanate Ester Polymers and Composites for Metal Plating" (Ser. No. 08/339,380) recites treating the surface with a preheated solution comprising a quaternary ammonium hydroxide or a primary amine.
The above-described surface treatment methods achieve greater adhesion for cyanate ester polymer composites by chemically etching the composite surfaces. Chemical etching textures the composite surfaces, thereby providing mechanical anchoring sites for the plating of metal such that the metal adheres to the composite. Scanning electron micrographs have revealed that surface texturing derives from microcracking in the cyanate ester resin that is wedged between the stiff graphite fibers of the composite.
In practice, however, the adhesion obtained for cyanate ester composites by the above-described surface treatment methods is not uniform across a composite surface. Although the fabrication of such composites involves blending the resin and graphite in an attempt to achieve a substantially uniform distribution of graphite throughout the composite, such composites invariably exhibit wide variations in graphite concentration across their surfaces. Accordingly, some regions of the composite surface are "resin-poor", and these areas may be over-etched by the above-described surface treatment methods. More particularly, etching "resin-poor" regions digests resin needed to glue the graphite fibers in place, such that the graphite fibers actually dissociate from the composite surface. This dissociation of graphite fibers weakens the mechanical anchoring sites necessary for adhesion of a subsequently-plated metal to the composite.
In sum, while the above-described surface treatment methods provide the surface texturing needed for mechanical anchoring sites on composite surfaces, the intrinsic heterogeneity of cyanate ester resin composites on a microscopic scale prohibits uniform surface texturing across the composite surface. Yet, good mechanical adhesion requires a dense and uniform distribution of anchoring sites for plating.
Thus, a need remains to achieve uniformity of surface consolidation of cyanate ester composites to ensure good plating adhesion. The adhesion must be achieved in a simple, easily-implemented manner that lends itself to reproducible results, since a single plating failure in an application such as a satellite microwave circuit can be economically catastrophic.